The in vivo mouse embryoid body/teratocarcinoma system is being utilized as a model to study the molecular mechanisms of cellular determination and differentiation and the effects of environmental agents on differentiating mammalian cells. The wheat germ S-30 and nuclease-treated rabbit reticulocyte lysate in vitro translation systems permit the analysis of the patterns of protein synthesis during the differentiation process from simple embryoid bodies to teratomas. In vitro translation systems aid in the characterization of total or polysomal poly(A)-containing mRNA by analysis of synthesized products. Isolation of specific mRNA's (e.g. alpha-fetoprotein (AFP, S-100, and glial fibrillar acidic protein (GFAP) will permit the analysis of gene transcription with cDNA probes during differentiation and the analysis of precursors of the final protein products. AFP is produced in increased quantities following partial hepatectomy, toxic injury to the liver, and hepatomas and teratomas. In vitro translation of mRNA's and nucleic acid hydridization with a specific cDNA to AFP mRNA will permit the study of the molecular events surrounding the expression of AFP during toxic injury and carcinogenesis.